Process for coating surfaces

ABSTRACT

The invention concerns a process for coating a metallic or semimetallic surface in which coating molecules containing reactive groups are bound covalently to the surface by irradiation with light and it also concerns a structured coated surface.

DESCRIPTION

[0001] The invention concerns a new process for coating a metallic orsemimetallic surface as well as a structured coated surface.

[0002] Surfaces can be functionalized by coating in order to achievepredetermined and desirable properties. The covalent coating of siliconsurfaces with organic molecules via a carbon-silicon bond is known (M.R. Linford et al.; J. Am. Chem. Soc. 115 (1993), 12631-12632;M. R.Linford et al., J. Am. Chem. Soc. 117 (1995), 3145-3155). These coatingswere produced by reacting hydrogen-terminated Si (111) with 1-alkenes bymeans of a radical mechanism using diacyl peroxides as radical starters.However, the use of peroxides is disadvantageous since peroxides areextremely reactive, dangerous to health and furthermore their reactivityhas a low specificity.

[0003] Furthermore it is known that self-assembled monolayers offunctionalized organosilyl compounds can be formed on hydroxylatedsilicon surfaces (F. Effenberger et al., Synthesis 1995, 1126-1130; K.Bierbaum et al., Langmuir 11 (1995), 512-518; S. Heid et al., Langmuir12 (1996), 2118-2120; P. Harder et al., Langmuir 13 (1997), 445-454).The covalent binding of silicic acid compounds to the OH groups of anoxidized silicon surface is described in EP-A-0 664 452. However, adisadvantage of this process is that the surface has to be chemicallypretreated before coating and moreover undesired cross-linking reactionscan occur. In addition it is advantageous for many applications when thecoating is applied to a metallic and not to an oxidized surface.

[0004] The object of the present invention was therefore to provide aprocess for coating metallic surfaces which does not have thedisadvantages that occur in the prior art.

[0005] The object is achieved according to the invention by a processfor coating a metallic or semimetallic surface which is characterized inthat coating molecules which contain reactive groups are boundcovalently to the surface by irradiation with light.

[0006] It was surprisingly found that activation with light enabled anefficient coating of metallic or semimetallic surfaces with moleculescontaining reactive groups to be obtained. In connection with thepresent invention the term “reactive group” means that the reactivegroup is covalently bound to a surface under suitable conditions andwhen irradiated with light of a suitable wavelength.

[0007] The binding of the coating molecules to the surface can be basedon a direct photoactivation of reactive groups in the coating molecules.Furthermore the binding can also be achieved by photoactivation of thesurface itself if this contains reactive groups e.g. groups that can beactivated by light e.g. metal or/and semimetal hydride compounds such assilicon hydride. The groups on the surface activated by light are thenable to covalently bind to reactive groups of the coating molecules. Acombination of both reaction mechanisms i.e. photoactivation of thecoating molecules and photoactivation of the surface is also possible.

[0008] The surfaces to be coated have metallic or semimetallicproperties and can comprise one or several metals, semimetals or/andmetallic or semimetallic compounds. Examples of suitable elements whichform a metallic or semimetallic surface are metals and semimetals ofgroups 3 to 16 of the periodic system. Particularly preferred examplesare silicon, germanium and metallic compounds that contain theseelements. Examples of surfaces which contain more than one element arealloys which comprise two or several metals or/and semimetals. Anexample of a compound which has metallic properties is gallium arsenide.The surface is particularly preferably a hydrogenated surface i.e. themetal or/and semimetal atoms of the surface layer are at least partiallybound to hydrogen. An example of a metal or semimetal hydride is siliconhydride. The metallic surfaces used according to the invention are inparticular surfaces which have non-oxidized elements or compounds.

[0009] According to the invention the molecules containing the reactivegroups are bound covalently to the metallic or semimetallic surface, forexample via a surface element-carbon or surface element-oxygen bond. Inthis connection the molecules used can basically contain any reactivegroup i.e. a group which can be converted directly or/and due tointeractions with the surface, into a reactive species and in particularinto a radical by irradiation with light. The reactive group ispreferably selected from C═C or C═O double bonds. Examples of suchreactive groups are alkene, aldehyde and vinyl ether groups. Aldehydegroups are particularly preferred which can be used to obtainsurprisingly high degrees of coverage. In the case of aldehyde groupsbinding to the surface is via the oxygen. In contrast to known coatingprocesses on hydroxylated silicon surfaces in which the thickness of theSiO₂ layer is undetermined, it is possible to obtain predetermined,defined and uniform coatings with the process according to theinvention.

[0010] When using alkene groups, the coating molecules are bound to thesurface via a carbon atom.

[0011] Coated metallic or semimetallic surfaces are produced by theprocess according to the invention which are suitable for numerousapplications e.g. to manufacture microelectronic components as well asfor applications in diagnostics and medicine. Applications forfunctionalized surfaces which have been produced by chemically couplingcoating molecules to the surface are described for example in WO92/10092, Fodor et al., (Nature 364 (1993), 555-556) and Cheng andStevens (Adv. Mater. 9 (1997), 481-483). Surfaces coated according tothe invention by photoactivation are also suitable for theseapplications.

[0012] For this purpose it is expedient to use molecules for the coatingwhich contain at least one additional functional group in addition toone or several reactive groups. The additional functional group ispreferably selected from haptens, biotin, chelating groups, nucleotides,nucleic acids, nucleic acid analogues, polyethylene glycol, conjugatedπ-systems, charged groups, non-linear optical structures, cross-linkablegroups, electrically conductive groups, amino acids, peptides andpolypeptides.

[0013] Molecules can be used for applications in microelectronics whichcontain conjugated π-systems. Especially in the case of such compoundsactivation with peroxides known in the prior art is disadvantageoussince the occurrence of undesired radical side reactions is observed. Incontrast by irradiating according to the invention with light of adefined wavelength it is possible to selectively activate those reactivegroups that are intended for binding to the metallic surface and at thesame time not to influence other groups e.g. π-bonds or conjugatedπ-bonds in the molecule. Polyenes, aromatics, e.g. polyphenylenes,heterocycles e.g. polyheteroaryls and/or polyacetylenes are preferablyused as organic molecules containing conjugated ρ-systems.

[0014] After application to the metallic or semimetallic surface thecoating can be cross-linked by means of cross-linkable groups such asdouble bonds e.g. acrylic esters or/and triple bonds. Cross-linking canalso be achieved by oxidation e.g. of sulphur-containing groups such asthiol or thiophene groups.

[0015] By using haptens, polypeptides and/or biotin it is possible toform solid phases that are capable of specific binding which can forexample be used in diagnostic methods e.g. immunoassays.

[0016] When the surface is coated with nucleotides, nucleic acids ornucleic acid analogues such as peptidic nucleic acids, surfaces areobtained that are for example suitable for nucleic acid hybridizationtests.

[0017] The use of polyethylene glycol or other inert molecules enablesthe production of chemically inert and resistant surfaces as well asbiocompatible surfaces e.g. for use as implant surfaces.

[0018] Electrically conductive groups and certain polypeptides such asbacteriorhodopsin are especially suitable for electronic applications.

[0019] The additional functional groups can be optionally blocked byprotective groups during the binding of the organic molecules to thesurface. However, such blocking is often unnecessary if the wavelengthused for the photoactivation is selected such that although thephotoactivatable group is activated, the additional functional groupdoes not react in the wavelength range of the irradiated light.

[0020] A metallic or semimetallic surface is preferably used whichcontains at least one element selected from B, Al, Ga, Si, Ge and As. Asilicon hydride surface is particularly preferably used as the surface.Such a surface can for example be produced by etching oxidized Si-(111)with ammonium fluoride (G. J. Pietsch, Appl. Phys. A: Mater. Sci.Process. A 60 (1995) 367-363; G. J. Pietsch, “Structur und Chemietechnologischer Silicium-oberflächen”, VDI “Fortschrittsberichte” series9, 148, VDI Publishers, Düsseldorf 1992).

[0021] The coating is preferably carried out in an inert gas atmosphere,for example under argon.

[0022] The light source used for irradiation in the process according tothe invention must have sufficient intensity to achieve an adequatecoating efficiency.

[0023] Basically any type of light source is suitable for the coatinge.g. lamps such as a HBO lamp. However, the use of lasers has proven tobe particularly advantageous. The light source is preferably able toradiate light of a defined wavelength or of a defined wavelength range.This can be achieved by using monochromatic light sources such as alaser or appropriate filters. When monochromatic light or light with anarrowly defined wavelength range is used, it is possible to selectivelyexcite the photoactivatable groups of the coating molecules or/and ofthe surface while other functional groups present in the coatingmolecules are not influenced.

[0024] Light is preferably used with a wavelength in the range 350 to400 nm, in particular 370 to 395 nm. It is particularly preferable toirradiate with light of a wavelength in the range 380 to 390 nm.

[0025] The irradiation period and intensity depends on the desireddegree of coverage, the metallic or semimetallic surface that is used,the coating molecules that are used and the reaction conditions. Usuallythe desired results are obtained by irradiating for a period of 1 minuteto 30 hours.

[0026] The process according to the invention produces a laterallyhomogeneous layer of coating molecules, preferably a monolayer, over thecoating area. In this process it is possible to use a single species ofcoating molecules and also a mixture of several species of coatingmolecules. In a preferred embodiment of the process according to theinvention, a mixture of different coating molecules is used in which onespecies of coating molecule contains a group e.g. biotin, hapten,polypeptide, nucleic acid etc., capable of binding specifically to afree reaction partner whereas the other species of coating moleculecontains no such functional group and thus acts as a diluent molecule.The molar ratio of functional coating molecules to diluent molecules ispreferably 1:100 to 100:1.

[0027] An important advantage of the process according to the inventionis that it is possible to selectively and precisely coat certain areasof a surface. For example a metallic or semimetallic surface can beselectively coated in very small spatial areas down to the micrometerrange by using suitable optics or by using laser technology. Thus forexample coating molecules which contain electrically conductive groupscan be applied to desired sites by irradiation to form a circuit thatcan be used in microelectronics. Of course it is also possible to coat alarge area of the surface. In addition to the use of lasers and/orfocussing optics, a structured coating can also be applied by usingmasks or/and optical gratings. It is possible in this manner to producecoated metallic surfaces with any predetermined patterns. The processaccording to the invention is especially suitable for producing arraystructures which can for example be used in diagnostics to manufacturebiosensors.

[0028] The application of molecules which have groups capable ofspecific binding such as haptens, biotins, nucleobases, nucleic acids,nucleic acid analogues, polypeptides etc. enables the production ofcoated surfaces for diagnostics. Surfaces which are composed of amaterial that is usually rejected by the body can be made biocompatibleby using biocompatible organic molecules.

[0029] A further subject matter of the invention is a coated objectcomprising a carrier with a metallic or semimetallic surface andcovalent molecules which are covalently bound thereto which ischaracterized in that the surface has an arrangement of coated anduncoated areas. The coated areas are preferably miniaturized i.e. theyhave an area of 1 μm² to 10 mm², particularly preferably 10 μm² to 1mm². In contrast to the processes known in the prior art for coatingmetallic surfaces by chemical activation in which only a coating of thewhole area was possible, the process according to the invention enablesa structured coated surface to be obtained. A surface with a definedcoating pattern can be produced in which the coating is only applied topre-determined areas of the surface. The surface preferably has an arraystructure. Suitable focussing optics and/or laser techniques enableprecise locally delimited areas to be selectively coated. The surfacepreferably comprises coating molecules which contain at least oneadditional functional group or mixtures of functional coating moleculeswith inert diluent molecules. Depending on the application it ispossible to use appropriate functional groups as described above.

[0030] The invention is further elucidated by the attached figures andthe following examples.

[0031]FIG. 1 shows compounds 1 to 3 which were used to coat metalsurfaces. The compounds 1 a, 1 b and 1 c are alkenes, compound 2 is avinyl ether and compounds 3 a and 3 b are each an aldehyde.

[0032]FIG. 2 shows schematically the formation of a layer of octadecanal3 a on a H-terminated Si-(111) surface.

[0033]FIG. 3 shows the dependency of the degree of coverage of atetradecanal layer on an Si surface on the irradiated wavelength.

[0034]FIG. 4 shows the dependency of the degree of coverage of anoctadecene layer on an Si surface on the irradiated wavelength.

EXAMPLES Example 1 Coating of a H-Terminated Silicon Surface withAlkenes and Vinyl Ethers

[0035] An Si-H surface was produced by etching oxidized Si(111) waferswith ammonium fluoride according to known methods (G. J. Pietsch, Appl.Phys. A: Mater. Sci. Process. A 60 (1995) 347-363; G. J. Pietsch,“Struktur und Chemie technologischer Siliciumoberflächen”,“VDI-Fortschritts-berichte”, series 9, 148, VDI Publishers, Düsseldorf1992). The silicon wafers (Si(111)p-type) were obtained fromWacker-Chemitronic GmbH and Siltronix.

[0036] The H-terminated silicon surface etched with fluoride ions wasadded to the pure compounds 1, 2 or 3 (cf. FIG. 1) in a glass cuvetteand irradiated for 20 to 24 h in an inert gas atmosphere. An optimalreaction time of 20 to 24 h was determined under the experimentalconditions used here whereby shorter or longer reaction times result ina reduced coverage. The formation of self-assembling monolayers wasinitiated by irradiation with a HBO lamp (150 W). Then the siliconsubstrate was removed, rinsed several times with dichloromethane andwiped with cotton in order to remove physically adsorbed material.

[0037] As a comparison the formation of monolayers containing1-octadecene (1 a) on a H-terminated Si(111) surface was initiated bydioctadecanoyl peroxide according to the prior art (M. R. Linford etal., J. Am. Chem. Soc. 117 (1995), 3145-3155). The results are shown inTable 1. TABLE 1 Contact Reaction Tempera- Cover- angle time tureν_(as)(CH₂) age θ_(a)(H₂O) Reactants (hours) (° C.) (cm⁻¹) (%) [a](degree) [C₁₇H₃₅C(O)O]₂ 1.5 100 2918.7 50.5 91 [C₁₇H₃₅C[O])O]₂/ 1.0 1022918.6 57 96 1a (1:1) [C₁₇H₃₅C(O)O]₂/ 1.5 101.0 2920.0 48 91 1a (1:9 OTS15 20 2917.7 100 106  1a 24 20-50 [b] 2920.5 55 95 1a[c] 20 20-50 [b]2922.4 21 — 1b 13 20-50 [b] 2920.0 40 — 1c 20 20-50 [b] 2921.6 29 — 2 2030 2921.0 45 80

[0038] As can be seen in Table 1 the process according to the inventionenables approximately the same degrees of coverage to be obtained foralkenes as processes using peroxides known in the prior art but withouthaving to use these hazardous and unspecific chemical reagents.

[0039] The surface coverage was calculated using equation 1:

y=0.007x−0.03429(x=number of CH₂ groups)

[0040] which was obtained by linear regression analysis of the diagramareas of the ν_(as)(CH₂) and ν_(s)(CH₂) bands of sevenalkyltrichlorosilanes (wafer data) in relation to their chain length (8,10, 12, 14, 16, 18 and 20 CH₂ groups) (S. Heit et al., Langmuir 12(1996) 2118-2120). The coverage is obtained by dividing the areas of thebands of ν_(as)(CH₂) and νs(CH₂) by y.

[0041] In addition to the terminally non-substituted 1-alkene 1 a, themonolayer formation was also examined using the pureω-thienyl-substituted 1-alkenes 1 b and c and the hexadecylvinyl ether 2by irradiation under comparable conditions. Moreover it was found thateven a low dilution of the 1-alkenes with an alkane as a solventconsiderably influences the formation of the self-assembling monolayerwhich results in a low coverage. The use of a mixture comprising nineparts of compound 1 a and one part hexadecane for example has a degreeof coverage of only 20%.

Example 2 Coating of a H-Terminated Silicon Surface with Aldehydes

[0042] The experiment described in example 1 was carried out under thesame conditions as described for compound 1 a except that pure1-octadecanal (3 a) (R. Redcliff et al., J. Org. Chem. 35 (1970),4000-4002) was used as a coating molecule. It was established by highresolution AFM images (50×50 nm²) that a homogeneous self-assemblingmonolayer is obtained with a high degree of coverage of 97% relative to100% coverage with octadecyltrichloro-silane (OTS). A monolayer ofcompound 3 a is shown schematically in FIG. 2. A mean angle ofinclination of 12° for the molecules in the monolayer relative to thesubstrate can be derived from the dichroic ratio D=0.54 (A. Ulman, AnIntroduction to Ultrathin Organic Films, Academic Press, London 1991).This result corresponds to the literature data for OTS films (M. R.Linford et al., J. Am. Chem. Soc. 117 (1995) 3145-3155.

Example 3 Formation of a Self-Assembling Monolayer Using DifferentWavelengths

[0043] Pure tetradecanal (compound 3 b) was applied as described inexample 1 on a Si(111) H surface at 40° C. at various radiationwavelengths. An XBO lamp (450 W) and a double monochromator (Jarrell AshCo. mod. 82-440) with 1.3 nm half width was used as a light source. Theresults are summarized in Table 2 and FIG. 3. TABLE 2 Degree ofWavelength Irradiation ν_(as)(CH₂) Area coverage (nm) period (h) (cm⁻¹)ν_(s) and ν_(as) (CH₂) (%) [a] 355 15.5 2922 0.0297 46 365 16.5 29250.03468 53 375 16.8 2922 0.03069 48 380 14.8 2921 0.03652 57 385 16.42920 0.0656 103 395 16.0 2922 0.03207 50 405 14.5 — — 0 415 15.0 — — 0

[0044] As seen from Table 2 and FIG. 3 the maximum degree of coveragewas obtained with light of a wavelength of 385 nm.

[0045] The dependency of the degree of coverage on the wavelength wasexamined analogously using pure octadecene (compound 1 a). Also in thiscase a maximum coverage at 385 nm was obtained (cf. FIG. 4).

Example 4 Formation of a Self-Assembling Monolayer of Aldehydes inRelation to the Irradiation Period.

[0046] The rate of formation of self-assembling monolayers was examinedwith pure octadecanal (compound 3 a) on a Si (111) H surface. Theinfluence of the irradiation period on the degree of coverage andorientation of the monolayers that are formed is summarized in Table 3.TABLE 3 Irradiation Degree of period ν_(as)(CH₂) area coverage (min)(cm⁻²) ν_(s) and ν_(as) (CH₂) (%) [a] 1 2922 0.0357 39 3 2918 0.0746 825 2917 0.0943 103 30 2916 0.0975 106 60 2916 0.0978 107 120 2916 0.083391

[0047] The H-terminated Si surface prepared as described in example 1was added to pure octadecanal (compound 3 a) in a glass cuvette. It washeated to 70° C. and irradiated with polychromatic light for the periodsstated in Table 3. The surface was removed and purified as described inexample 1.

[0048] After one minute irradiation period a degree of coverage of 39%was found and the position of the ν_(as)(CH₂) band at 2922 cm⁻¹ shows alow order of the monolayer. The degree of coverage was increased to 80%after 3 minutes. Already with an irradiation period of 5 minutes it waspossible to obtain an optimal degree of coverage of 103%. Theν^(as)(CH₂) band at 2917 cm⁻¹ indicates the formation of perfectlyordered self-assembling monolayer of compound 3 a.

Example 5 Preparation of Laterally Structured Coated Surfaces

[0049] In previous investigations on the structure of self-assemblymonolayers (Ulman, Chem. Rev. 96 (1996), 1533-1554 and furtherliterature references; Xia and Whitesides, Angew. Chem. Int. Ed. Engl.37 (1998), 550-575; Effenberger and Heid, Synthesis 1995, 1126-1130;Bierbaum et al., Langmuir 11 (1995), 512-518; Heid et al., Langmuir 12(1996), 2118-2120; Harder et al., Langmuir 13 (1997), 445-454) it hasnot been possible to obtain a direct lateral structuring of the surfacessince one started with chemically uniform surfaces and the subsequentchemical reaction with the layer-forming substrates also proceededuniformly.

[0050] However, the light-induced preparation of self-assemblymonolayers enables a direct structuring of surfaces by using suitablemasks. In this case foils that are used in the production of printedcircuit boards are particularly suitable as masks.

[0051] The structures of the masks were drawn with the computer programFreehand 5.0 and directly printed with a laser-assisted photoset system(Lintotronic 530, Linotype-light, resolution 2540 dots/inch) on polymerfoils (Fuji laser recorder film F100). Then the SiH substrate wascovered with the mask, octadecanal was added and melted. The monolayerformation and purification was carried out as previously described.

[0052] On the basis of IR spectra it was possible to show that when apart of the Si(111) H surface is covered only the non-covered arearesults in a SAM layer. Moreover it was found that the non-exposed partof the SiH surface is still fully reactive.

[0053] Due to the different wettability of coated or uncoated silicon itis also possible to visualize the structuring of the silicon surface byobservation with the naked eye or in a microscope. For this purpose thesubstrate provided with a structured coating was wetted under amicroscope (Carl Zeiss, twenty-fold enlargement) with a few drops of amixture of 2-propanol and universal oil (Lubricant Consult GmbH). Thepattern that formed after a few seconds was photographed.

1. Process for coating a metallic or semimetallic surface, whereincoating molecules which contain reactive groups are bound covalently tothe surface by irradiation with light.
 2. Process as claimed in claim 1,wherein the binding of the coating molecules to the surface is achievedby (a) a photoactivation of reactive groups in the coating molecules,(b) a photoactivation of reactive groups on the surface or (c) acombination of (a) and (b).
 3. Process as claimed in claim 1 or 2,wherein the reactive groups of the coating molecules are selected fromC═C or C═O double bonds.
 4. Process as claimed in claim 3, wherein thecoating molecules contain reactive alkene, aldehyde or/and vinyl ethergroups.
 5. Process as claimed in one of the previous claims, wherein thecoating molecules contain at least one additional functional group inaddition to the reactive groups.
 6. Process as claimed in claim 5,wherein the additional functional group is selected from haptens,biotin, chelating groups, nucleotides, nucleic acids, nucleic acidanalogues, polyethylene glycol, conjugated π-systems, charged groups,non-linear optical groups, cross-linkable groups, electricallyconductive groups, amino acids, peptides and polypeptides.
 7. Process asclaimed in claim 6 wherein the coating molecules contain conjugatedπ-systems as additional functional groups.
 8. Process as claimed inclaim 7, wherein the coating molecules contain polyene, aryl, heteroarylor/and polyacetylene groups.
 9. Process as claimed in claim 7 or 8,wherein the additional functional groups are blocked by protectivegroups during binding to the surface.
 10. Process as claimed in one ofthe previous claims, wherein the surface comprises at least one elementselected from B, Al, Ga, Si, Ge and As.
 11. Process as claimed in one ofthe previous claims, wherein the surface comprises at least one metalor/and semimetal hydride compound.
 12. Process as claimed in claim 11,wherein the surface contains silicon hydride.
 13. Process as claimed inone of the previous claims, wherein the coating is carried out in inertgas atmosphere.
 14. Process as claimed in one of the previous claims,wherein light of wavelengths in the range of 370-395 nm is irradiated.15. Process as claimed in claim 14, wherein monochromatic light isirradiated.
 16. Process as claimed in one of the previous claims,wherein a mixture of different species of coating molecules is used. 17.Process as claimed in claim 16, wherein one species of coating moleculescarries an additional functional group and another species of coatingmolecules carries no additional functional group.
 18. Process as claimedin one of the previous claims, wherein a structured coating is appliedby using masks or/and optical gratings.
 19. Process as claimed in claim17, wherein array structures are applied.
 20. Process as claimed in oneof the previous claims, wherein a coated surface is produced for use inmicroelectronics in which binding molecules which have electricallyconductive groups are applied.
 21. Process as claimed in one of theclaims 1-19, wherein a coated surface is produced for diagnostics inwhich binding molecules are applied which have groups capable ofspecific binding.
 22. Process as claimed in claims 1-19, wherein abiocompatible coating is applied.
 23. Coated object comprising a carrierwith a metallic or semimetallic surface and coating molecules covalentlybound thereto, wherein the surface has an arrangement of coated anduncoated areas and the coated areas are miniaturized.
 24. Object asclaimed in claim 23, wherein it is a biosensor and has an arraystructure.
 25. Object as claimed in claim 23, wherein the coatingmolecules contain at least one additional functional group or aremixtures of coating molecules with or without additional functionalgroups.